text:
Solutions to improve the aerodynamic behavior of a UAV
So I'm working at a project of a UAV with the follow characteristics:

Rectangular planform
Conventional tail
The fuselage is just a metal/aluminum rod

So we need to improve its aerodynamic behavior/perfomance.
I thought about: Winglets, big AR, a tapered planform or an elliptical one for reduced induced drag, maybe flaps/slats. I cant think anything about the tail actually.
The UAV is similar to the one in the picture. (The tail and wings)

What are the objectives of the UAV? Factors that "improve" performance depend on what kind of performance you're looking for.

@fooot Well, we can say that the UAV is going to be constructed for competitions that need cargo to be moved around for some time, we dont need high power as it will fly at low speeds like 50-60km/h.
We want it to be stable enough for long flights. The purpose of the UAV is not clear yet. The question is more generic about what would someone suggest. I know that you also need some data and calculations to be done, but we are talking about the very first phase of the design.

Generic answers don't make any sense, because the improvements you can make oppose each other: For example, improving STOL performance may hurt cruise speed.  That's why people want to know what you want to improve.

@Dave, Bianfable, bogl, fooot: I was just in the middle of writing an answer. May I politely disagree with your judgement?

I have nominated this to be reopened.  The comments are valid, but don't block others from answering.

This is a  really broad question. you may define what metrics you what to improve  (improving L/D ratio --which help travel farther e.g. minimizing energy used to travel a given distance-- may be counterproductive if you want to minimize the best climb speed --which can help stay aloft while minimizing speed and such hourly consumption--) and put this in the question description, not in comments only.

You want to move a heavy payload at low speeds. This means that induced drag will dominate the aircraft. If we assume a quadratic polar, the least amount of energy for staying airborne is needed when induced drag is three times higher than friction drag. Assuming a quadratic polar, the optimum lift coefficient at that point is $$c_L=\sqrt{3\cdot c_{D0}\cdot\pi\cdot AR\cdot\epsilon}$$
Obviously, you want a small zero-lift drag, but when you combine this with the high aspect ratio for a very efficient configuration, your lift coefficient will also become very high. You need to be careful to pick the right airfoil, and now Reynolds number effects demand to be observed. Once I know the overall size of your UAV, I can become more specific.
Just an example: If the zero lift drag coefficient is 0.02 and the aspect ratio is 12, the ideal lift coefficient is already 1.5. This is beyond most airfoils at Reynolds numbers below 1,000,000 and indicates a too high aspect ratio. Better add some chord in that case.
Also, some design decisions will look counterintuitive at first. I would strongly encourage you to add wing struts, like in a Cessna 172 or the Dedalus human powered aircraft. They help to save wing weight and thus reduce induced drag.
Don't overdo it with an elliptic planform! This will produce too small a chord on the outer wing and give the design undesirable stall characteristics. Better keep some chord and twist the wing so you will achieve the desired elliptical distribution at the design angle of attack. Better yet, save some wing weight by moving to a more triangular lift distribution – however, for small aircraft the optimum looks still quite elliptic.
The model pictured in your question has a lot of dimples on the fuselage - make sure yours will have a smooth surface.
Also look into improving propulsive efficiency. Use low RPM on a large propeller, if possible. You didn't mention your means of propulsion, so again it is hard to be more specific. Judging from the model, you will use electric propulsion. Use a brushless external rotor with a high pole count (low RPM per Volt).
So please provide more details and then you will receive a better answer.


text:
Would airline got charge for late on depart from the gate due to passenger boarding issue? If so, what is the charge like?
Airline wants the boarding be completed as quick as possible.  I wonder what is the cost for any delay? On the other hand, I want get a dollar value for any delay and see the justification value for any improvement.  So, I wonder airline would get additional charge for the gate on delay ?
Thanks.

What kind of cost/charge are you thinking of? A charge/fine from the airport? A charge from a regulatory body? Payments to passengers for delivering them late? More clarity in your question will help you get a better answer.

Chain of delays
Being late on departure is likely to cause you to be late on arrival, this has the following immediate consequences:

you may be delayed even more on arrival in congested airports (e.g. London) if you miss your scheduled landing timeslot, being forced to circle around for, say, 20-30 minutes wasting fuel and time until you're allowed to land; I have been on a flight like that.
the main immediate effect is that some transferring passengers may be delayed because of this, which may possibly result in costs to you to reroute or compensate them.
you likely have scheduled the same airplane for the next flight with a minimal turnaround time; and delayed arrival may easily cause your next flight of that same airplane to be delayed (again, I have personally seen delays caused by this)
you likely have scheduled the same crew for the next flight, and in certain situations that delay may push them over the allowed work/rest schedule, so they are prohibited to fly their next (last in their shift) flight - so you need to rapidly find a new crew for the scheduled flight or cancel it.

For these reasons, a single delay sometimes might cause a chain of other delays or even cancellations, and become quite expensive.

You may highlight in your answer the part answering the question (would airlines got charged?). All the information you bring to justify avoiding late departure is useful but you may precise how and  if possible how many an airline get charged for all these delays (reroute passengers and crew may be an obvious one, staying at gate is not)

i wonder airlines are interested on idea to streamline the boarding process to avoid the potential delay.
As a passenger, would it be nice if we can just walk in without worry about room for the carry on or step over an aisle passenger already on his/her seat.

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Why do Ryanair pilots get paid such a low wage?
As I‘m living in Europe and I‘m interested in a commercial pilot career, I looked up the wages on various websites. (I hope the German screenshots don‘t matter too much).
Example 1 (Source)

Example 2 (Source)

Were/Are there really Co-Pilots who paid for their training themselves (I suppose) and now only get about 2000€ per month?

It surprises me to see that Easyjet pays about twice as much as Ryanair...

@Bianfable That was in fact one of the main reasons why I posted this question.

It's Ryanair.  What did you expect?

If you can, look up the wages of pilots working at feeder airlines to the (ex-) flag carriers. They get about the same pitiful compensation. The princely wages of some captains are a relic of the past.

Note that a new user who claims to be a "Ryr senior captian" posted a (now deleted) answer that I will reproduce as comment here: "25k? That's completely false... A Ryr senior FO makes 72k based on 850h FT, a captain makes 140k". If this is true, the Ryanair numbers are less off the average than shown in the table.

If my understanding is correct, @Andre is implying that somehow the wages quoted above for Ryanair, and Ryanair only, are for their training program, rather than their normal wages. If so, your sources screwed up

See this question: https://skeptics.stackexchange.com/questions/4845/do-commercial-airline-pilots-make-around-19-000-25-000-per-year

Before the recent pilot shortage started to really take effect this was fairly common around the globe and is still prevalent in some places. Low Cost Carries like Ryanair pinch pennies across the board to offer cheap flights. This in turn created a segment of the market that the major airlines (at least in the US and presumably elsewhere) took advantage of. Major airlines focused on longer haul international routes flying big metal and left short hop flights to either regional airlines or smaller affiliates which often conform to the low cost model.
However the big planes still needed pilots, so the low cost carriers became feeder programs for their bigger airline counterparts. Part of the reason Low Cost airlines can offer such low wages is that generally pilots don't work there that long. Much like flight instruction or banner towing it has become a way to effectively build hours to fly bigger planes. In some cases Low Cost Carriers also offer cadet/training programs that may undercut the cost to train pilots. Instead of taking it out as debt against the pilot then garnishing wages later they simply pay less.
Recently pilot wages (at least in the states) have seen a healthy increase to attract new people to the profession.

Your point about the pilot shortage holds true in this more recent graphic by the German Pilot labor union: source This graphic from 2018 sees new Ryanair Co-Pilots at a salary of approx. 40k € per year.

Yes.  The average entry level salary for a regional FO was about  $16,000.   It’s now slightly over twice that. That is a considerable improvement.  However when you consider that these people will probably have invested somewhere in the order of  $100,000 -$130,000 into flight training and ratings  to end up with a job with such little pay and low return on investment (at least initially), its no wonder why people have abandoned the pilot profession in search of more lucrative opportunities.

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Why was the "Dispatch Reliability" of the A320 "low" during its first years?
I found the following Statement on the German Wikipedia page about the A320 family of aircrafts.

Nach erheblichen Schwierigkeiten in den Anfangsjahren erreicht die A320-Familie mittlerweile eine Abflugzuverlässigkeit von 99,7 Prozent.
  (https://de.wikipedia.org/wiki/Airbus-A320-Familie#Wirtschaftliche_Aspekte)

This roughly translates to:

After significant difficulties in its early years, the A320-family reaches a Dispatch Reliability of 99.7% nowadays.

I could not find more information about this statement, so my question would be why the Dispatch Reliability of said aircraft family was supposedly significantly lower in its early years? Is it normal for completely new types of aircraft to have such a low DR that it's worth mentioning?

The A320 was innovative in a number of ways, especially its Fly By Wire control system, and now it is an incredibly popular and mature platform, partly because of its despatch reliability.
All the elements that allow a despatch of the aircraft for a commercial flight contribute to despatch reliability. This can be anything from having spare parts for anything that needs to be replaced, to crew being familiar with inspection and preparation procedures (checklists) for the aircraft, to pilots correctly understanding messages on the flight management system.
When aircraft are first introduced, there are often problems with logistics and the (unintended) interactions between aircraft systems. As the aircraft accumulates more operating hours, these problems get solved. As aircraft reliability improves over time, it can also be safe to operate in "MMEL" conditions (master minimum equipment list).
For example, if you know it is safe to despatch with a certain warning message because you have proven that the 3 redundant backup systems are functioning properly - this further helps despatch reliability, because now you know it is safe to dispatch whereas on day one with the new aircraft, you did not have that option.
When I worked as a supplier to two major airframers, they were always incredibly keen to improve despatch reliability by fractions of a percent.


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What aircraft is this clock from?
I have come into possession of some of my grandfathers things including some aircraft parts that he had. He was a naval pilot who flew single engine fighters in WW2 and then at some point transitioned to flying helicopters (some sort of sikorsky) which he piloted in the korean war.  After he retired from the navy he continued to fly as a civilian contractor for the air force until around the 1960s.  I imagine this came from one of those aircraft.
This appears to be a clock.  The knob on the bottom left winds it and it will still operate but barely and only when sitting almost perfectly upright.  Turning the wind knob to its limit doesn't even last 24 hours though which I suppose would be perfectly acceptable for a flight operation.  The knob on the bottom right switches the day/night indicator.  I am not sure what the "stop/go" switch does.
I have a few questions about this clock if that is allowed:

What aircraft is it from?
Since it can be wound mechanically what is the purpose of the plug on the back?
What does the "stop/go" switch do?

If you are able to move the stop/go selector to another position I suspect that there is a "0" behind it. Based on the description in the answer that you received the 0 position is used to reset the clock, which is done by pressing the button in the lower right. In the catalog photo this button is marked "Push Hard". In your photo the button is unmarked, perhaps due to years of being pushed hard, assuming that the button performed the same function as the other clocks.

@StevePemberton: Possibly but that little circular window at the bottom has an indicator that is either white, half white half black, black, or half black half white.  I'm assuming to indicate day, evening, night, dawn.  Pressing the button in the bottom right seems to only change this indicator.  The clock also runs when on stop, 0, or go.

Here's an excerpt from page 13 of the manufacturer's 1966 catalog for a very similar clock, model 651-12-24.

It might have been installed in many aircraft.  Logos on page 14 include American Air Lines, KLM, Cessna, Piper, United Air Lines, BOAC, USN, Lockheed, Beechcraft, PAA, Douglas, and USAF.

The manufacturer's catalog mentions "bulbs-5 volt filament / standard connectors-to customer's specification," so the plug on the back must be for its lightbulb(s).  (This is also from page 13, just above the excerpt.)  The connector is like the 3-pin XLR sometimes still used for lighting, e.g. Littlite goosenecks for audio mixing consoles, but its pins are spaced differently.

When the stop/go switch points at stop, the clock does not run.  When it points at go, the clock runs.  The excerpt's description begins:  "Operation: Start, stop, ...".  So it's not intended to synchronize to your wristwatch;  it's more like a stopwatch, which happens to be able to measure durations of many hours.


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Why do we use composite aluminium or titanium stacks in airplanes?
Why do we use composite aluminium or titanium stacks in airplanes? What are their machining conditions?

I rephrased your question a bit and expanded the acronyms (like "/al" and "/ti") you used. I hope this is what you meant to ask. If not, please edit again to clarify.

This is more accurate , thanks

What do you mean by 'stacks'? Does 'machining conditions' mean that you want the feed rate and tool speed?

I've boldfaced the parts of this abstract that answer the question.

Abstract: Ever since the advent of fiber reinforced polymer materials in the field of Aerospace, Metal-FRP stacks started to gain importance due to their superior fatigue performance, phenomenal low weight and good specific strength. However the machining, specially drilling these multi stack materials has always proved to be a challenge ... The very properties that make the FRP composite superior make it a challenge for machining. The mechanism of machining viewed here is more or less fracturing and breakage of the fibers present in the system. This breakage of fiber also induces fractures in the tool resulting in accelerated tool wear. These characteristics render the process unreliable. Also the temperature during the machining of composites is higher than while machining a metal due to the lower thermal conductive properties of the composite. When two such extremely different elements are brought together in the form of a FRP-Metal stack, various challenges are faced to achieve required hole size, quality, tool life, ...

-- Peter Mueller-Hummel, Prithvi Sripathy, and Abdelatif Atarsia, "Fail Safe Drilling of CFRP/Titanium and /Aluminium Stack with H8 Quality for Aerospace Applications," SAE International, 2013.
Cutting speed, cut depth, and feed rate depend strongly on the particular stack's composition: which metals, which composites (carbon, aramid, fiberglass), their ratio, each layer's thickness.

Metal/composite stacks are used all over an airframe.  The 2011 article "A review: Fibre metal laminates, background, bonding types and applied test methods" by Sinmazçelik et al calls them FML's, fiber metal laminates, and figure 3 from there shows where they're found on the Airbus A380:

Oh, it's allmost as if the answer begged the question :)

Would you please explain where the stack is on an airplane?

thanks for your answer . I mean, why aren't these substances used separately? What is the reason for composite and aluminum sheets to stick together?

To combine the properties of both materials, however is needed for that part: stiffness, tensile strength, load transmission, corrosion resistance, heat conductivity.

What causes defects in composite and aluminum plates during drilling operations?

That's a different question, and probably one for a non-aviation StackExchange site.

ok thanks , Can I have your email address?

If you ask on stack exchange instead of asking one person, you're much more likely to get an answer from an expert.

Aluminum and titanium are used in aircraft because they are light weight materials that are strong and durable like steel, however they are non-ferrous unlike steel meaning no rust. The two metals are usually composited with another material such as carbon, composites are known for being stronger than using one base material because it will alter the chemical or physical properties. This makes them ideal for stacks in the aircraft because of their properties. And I don't understand what you mean by machining.


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Why do aircraft cockpit displays use uppercase fonts?
I've seen many images of aircraft cockpit displays and instrument or control labels that all seem to use uppercase lettering, even though there seems to be a lot of evidence indicating that mixed case lettering has superior readability. Why is that?

Welcome to aviation.SE! This might be an interesting question for ux.SE if you don't get a good answer here

The fonts may have only uppercase, but you can't tell by the display.  They only use uppercase, but the font itself may well contain the lowercase letters - as well as other characters.

UPPER CASE HAVE A UNIFORM HEIGHT SO THEY ARE EASIER TO LINE UP WHEN YOU PAINT THEM?

At the very least, if you see "I" you know that's an i and not an L

Not all avionics systems use uppercase exclusively. You can find lowercase in some of Garmin's recent offerings, for instance. See for instance the lowercase labels for the buttons in this image of a G1000. But for much of what is displayed, lowercase doesn't make a lot of sense.

@Pondlife I found an interesting read on the topic (abstract/introduction): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2016788/

For at least certification under the FAA via AC 25-11B all upper case is acceptable but not explicitly required. If you're building an instrument and you know that all upper case will be accepted during certification it's only logical to build it that way.

5.4.2  Regardless of the font type, size, color, and background, text should be readable in all foreseeable lighting and operating
  conditions from the flightcrew station (§ 25.1321(a)). General
  guidelines for text are as follows:
5.4.2.1    Standard grammatical use of upper and lower case letters is recommended for lengthy documentation and lengthy messages. Using this
  format is also helpful when the structure of the text is in sentence
  form.
5.4.2.2    The use of only upper case letters for text labels is acceptable.

However this NASA study finds that mixed case is actually more legible. But this seems to be applied to lengthly sections of text.

There is almost a consensus among researchers that, when other factors
  are controlled, lower-case characters are more legible than upper-case
  (Hartley, 1981; Philips, 1979; Tinker, 1963). Poulton (1967) performed
  an experiment to determine the difference in readers’ attention
  between upper and lower-case in newspaper headings. He reports that
  lower case headings were located faster than upper case heading.
  Tinker (1963) tested lowercase and upper-case fonts for legibility and
  pleasingness. He reported that lower case was read faster and ranked
  higher in pleasingness.

This FAA paper suggests that all-capital letters for single words may be easier to identify

Although continuous text is easier to read when presented in mixed
  case versus all upper case, single words may be recognized better when
  displayed in all upper case (AC 25-11A; Ahlstrom and Longo, 2003).

Which may explain why it's used for instruments. They quote AC 25-11A however the language of that AC does not suggest this. But Ahlstrom and Longo, 2003 has some great info on the topic specifically 8.2.5.8 Capitalization specifically subsection 3:

8.2.5.8.3 Use of capitals. Capitalization should only be used for: headlines, key phrases or acronyms, short items to draw the user’s
  attention to important text (for example, field labels or a window
  title), the first letter in a sentence, or a single character in each
  word in a title or label. [Source: National Air Traffic Services,
  1999]

I would check out the whole document for more info.

by far the best answer here! Well reasoned with references to back it up. Cheers Dave

The specific reason all-uppercase was adopted is just historical. Traditional steam gauge faces use capital lettering exclusively. The reason for that is that capital letters are easier to produce with limited tooling. When they were installed in aircraft, this cost-efficient convention was followed.
This pattern is not exclusive to gauges: historically, in Indo-European languages, uppercase letters came first. Bicameral script is a later invention, present only in a few (albeit very popular) language scripts.
Early typewriters were all-uppercase. Upper case persisted in teletype communications and early computers. Mixed case always added complexity, so its adoption in tech took time throughout the 20th century. This made all-uppercase text very familiar to the technically-minded people involved in aviation, right at the time when its conventions were being established.
The reason uppercase hasn't been replaced, even in glass cockpits, is a simple case of getting the best size:readability ratio. Capitals are fixed height and allow for twice larger letters (thus more readable) within the same amount of space.
For the same reason, exit signs in buildings spell "EXIT", not "Exit", most book covers feature all capital letters, and "NO STEP" won't be replaced with "Please avoid stepping here" anytime soon.
Mixed case has superior readability for natural English, but the language used in aviation is a specific lingo with heavy use of abbreviations. The word-recognition advantage you use reading this post would not initially work in reading "ALT" or "HDG". These are new symbols to learn. Once learned, they become more familiar than the original non-abbreviated terms they stand for.
Aviation documentation is written in mixed case, but the use of uppercase in cockpits continues to be beneficial, as uppercase helps automatically emphasize the exact terms as seen in the cockpit and elsewhere.
Will aviation displays ever switch to mixed-case? The process is ongoing, but rather than changing the old function, it's just the use of mixed-case for some new functions. These involve incorporating paperwork into the glass, which uses more natural language, for which mixed case is suitable (and, as Gerry correctly adds, officially recommended).
Aviation-specific, abbreviation-rich jargon is an artificial language, not a natural one, so there's no case for it to switch.

The majority of text on the PFD and ND are labels. The primary information is the numbers and the symbols. As you point out, labels from the steam gauge days were in all caps and that carried over as pilots don't like change. Current FAA guidance in AC 25-11B is that all caps for labels is acceptable and text in sentence form should be in mixed case.

Even in "glass cockpits", all upper-case may have an advantage. Text rendered on a raster display can be rendered legibly on a much lower resolution display when all upper-case. When mixed-case is used, higher display resolution is required to avoid artifacts that would impair legibility.

@AnthonyX True. I'm looking years ahead, when we'll have as much display resolution as desired. In that case, for natural language (I imagine actual paper-paperwork will eventually be gone completely), mixed case wins. It's not life-critical, it's done on the ground or in cruise, so it doesn't have to shout.

@Therac I should add that in the glass cockpit scenario, software complexity is a safety issue; more software or more complex software increases risk of bugs; lower-case lettering requires complex rules of typography as well as sophisticated "sub-pixel" rendering algorithms for optimal readability. All upper-case can use simple fixed-pitch or minimal kerning and no sub-pixeling and still be easily readable. That simplifies the software, increasing software reliability and provable correctness.

I wish I could pick multiple answers! What a cool community.

"Capitals are fixed height and allow for twice larger letters".... this can vary depending on the x-height of the font chosen: https://en.wikipedia.org/wiki/X-height

In low light or with vision partially obscured (like, say, smoke), all caps can be seen from farther away and/or with greater clarity. This is more important than the easier to follow at a glance word-shapes you have with upper and lower case text, which is only a significant benefit when words are strung into sentences that you scan.

Do you know of some research that backs this up? In the UK all road signs are in mixed case specifically for the readability reason, and they generally (obviously there are exceptions) aren't words strung together into sentences, but things like place names etc.

@Muzer Right - that's because the human brain can "read" text by looking only at the outline shape of the word instead of the letters - but this doesn't work if the text is all-uppercase (because then all words of the same length have roughly the same shape). I wonder if the all-uppercase trend started off as a hunch in the 1950s or so and by the time that solid research suggested mixed-case was better than all-uppercase there was too much institutional inertia to change? (I wonder if it could be because in WW2 almost all military stencil prints were all uppercase for simplicity...?)

@Muzer Really? UK stop signs spell "Stop" not "STOP" ?

@Muzer yeah, STOP and SLOW as well are examples where UK signs do use all-caps. I think the point is, single short words are better readable in caps, whereas place names are often long enough for the “shape” aspect to help, like it does in sentences.

@StianYttervik I think that might be because of the Vienna Convention on Road Signage.

@leftaroundabout good point, I didn't think of that. It's certainly nowhere near all road signs - a lot of them, though.

@Dai Use of mixed case in the UK predates the Vienna convention. It was introduced with the motorways in the 50s, and was based on German and American research on readability. See here

@Muzer "all UK road signs are in mixed case" is completely WRONG. In fact almost all MANDATORY signs are in upper case. Mixed case is reserved for additional information. See https://www.highwaycodeuk.co.uk/uploads/3/2/9/2/3292309/know-road-traffic-signs.pdf for many examples.

This answer really needs something to back it up.

I've read, (I don't have a reference) that mixed case is easier to read quickly, so I'm dubious about this answer.  All uppercase might just be a legacy from 1960s - 1980s military avionics which are slow to change.  My point is, military supplier when making products for civil use would use what they were familiar with.  Remember all those screens in the cockpit need device drivers -- it's a very different kettle of fish from writing to a pc screen.

You aren't reading labels for switches or titles in displays like a sentence.  It's a label.  It needs to stand out and be readable at a glance, from several feet away, and for a title of something the "word shape" doesn't really help you.

Actually, the Clearview people did the research and found that's wrong.  Mixed case with an appropriate font is correct.

Whatever. I'm just telling you why it's done in cockpits, having worked in the industry a long time.  Perhaps OEMs will start to change it.

@leftaroundabout (et al), I'd guess the difference is not so much the length, but rather the fact that place names (etc) you actually need to read (they are probably unfamiliar), whereas all-caps signs you just recognise. This latter is more applicable to the cockpit environment, where you generally know all the labels already. Also, many of the cocpit labels are (or contain) numbers, and so should be consistent in size with letters.

With no supporting evidence I simply don't believe this claim.

Bear in mind that YOU may be comfortable with lower-case letters, but there are other cultures in the world that have different symbols for their alphabets, and they have to learn the Latin alphabet as something new. So having only upper-case letters makes it easier for non-Latin users.
Note that this idea of all upper-case also applies to the QRH. For example, there is no such drill as "Fire Engine", it's "FIRE ENGINE" because the EICAS only shows upper-case messages, and so the pilot doesn't have to understand the message, they simply look through the QRH to pattern-match the EICAS message. If a drill/procedure in the QRH has an initial-case title, then it's for a non-EICAS procedure (usually called "Unannunciated").

Japanese Katakana has 46 characters - and written forms of Chinese have thousands of glyphs - while it's true that Latin has the uppercase/lowercase property distinction that other writing systems don't have, I don't think it's fair to assume that non-native English speakers wouldn't be "comfortable" with mixed-case writing. As for EICAS: I understand it's mostly uppercase because the system predates modern computer typography and because it was created in the atmosphere of legacy radiocommunications systems that were also all-uppercase.

Having started to learn a bit of Japanese script, I can tell you that having hiragana and katakana adds complexity. If I had to learn to read instruments in Japanese, I'm sure that having all messages in the same script (hiragana or katakana) would help. Therefore, it seems quite reasonable to expect people from other scripts to be more confortable with all caps than with mixed cases.

Hiragana and katakana, if they appear together, are used to make adjacent words look different - they're not like upper / lower case, they're more like roman and italic, say. So katakana may be used for emphasis of something, or as a marker for a foreign loan word. In this case using different scripts makes it hard to read something as a "word" ...

The "easier for non latin culture" explanation may be an a posteriori easy explanation. Historically, aviation is mostly US-habits adoption. In a all-english environment, all caps might have been choosen without taking into account non-latin culture, and then adopted as a de facto standard as many other US standard.

I have no true evidence that it is really related, but upper case only used to be the standard before the 80's. For example, METAR et TAF messages (meteorological observation and previsions on airports) used to be exchanged in old 5 bits Baudot code that can only represent upper case alphabets - and the code for both messages still only contains upper case Latin alphabets, decimal digits and spaces.
As security is very important in aviation, people are reluctant to change something that has never caused any problem. Long story short, people using planes are used for decades to the upper case latin alphabet whatever their first language charset is, and find no strong reason to change that.

I don't believe Baudot code specified letter-case. It does lack a way of encoding letter-case alternatives, though, so a given machine will support only one. Confusingly, capital letters were usually called "lowercase" (because they came from the lower teletype case) and figures and punctuation symbols "uppercase". The lowercase almost invariably used capital letters though. For example, 2 might be referred to as an uppercase W.

From a typography perspective, all caps does not have any descenders (letters that go below the baseline such as "p", "g" and "j".) It is easier to do labels and such when you don't have to worry about accommodating space for those. In other words, you can make the text bigger for a given space: "Eject" vs "EJECT" or "Flaps" vs "FLAPS"

I'd been going to add that when the reader is in a hurry, the lowercase l can commonly be mistaken for the uppercase I and then both can be confused with control/command characters like | which is commonly used as a column separator.  It's a particular issue with monospace and sans-serif fonts, which are typically what you get in digital displays.  I think this fits better alongside your answer instead.

I don't have any references to back this up, but to me it does make sense to have all upper-case letters for the following reasons:
1) Cockpits are usually packed with instruments which creates limitations on how large a letter can be printed. Considering even in normal circumstances the cockpit of an aeroplane can have a level of vibration, I believe having upper-case letters will improve the readability. This is because the upper-case printed letter is larger than its lower-case counterpart -- in oppose to having words written in Capital-case where most letters are written in lower-case.
In my personal experience in an environment that is subject to a noticeable vibration, the word "OFF" is more readable than "Off", so the word "NAV" is more readable than "Nav".
2) If for whatever reason the pilot's vision is reduced e.g. blurry vision due to not being able to use their glasses, an upper-case letter seems more readable, purely due its increased size but also for some letters because of their shape too. I myself can't distinguish between lower-case letters "r" and "i" from a certain distance without having my glasses on, but I have no problem recognising "R" and "I" from the same distance.
3) To me all upper-case words carry more importance over capital-case words. As an example in emergency situation e.g. if I am looking to exit a burning building, I believe my brain would notice "FIRE EXIT" much quicker than "Fire Exit", probably because upper-case words are not used in normal communication very often and visually they are distinctive to all the other text we read everyday.
In an emergency situation when priorities are different in my brain in terms of signal processing, I don't need a text saying "If you are involved in a crash, use this red hammer to break the window glass and then exit through the window." A minimal text such as "BREAK GLASS" would do way better in such a situation than a text that otherwise in normal day-to-day life situation would have been easier and better to read. This doesn't really answer this question, but I thought it is worth mentioning as I believe this may have also played a role in choosing upper-case words rather than capital words.
And again, all of these are my personal opinion and I have nothing to back them up.


text:
Quality indicators ADS-B out & Ground Station receiver
I have a question about quality indicators parameters:
I discussed with a pilot about quality indicators .
We talked about NIC and NACp parameters
I received his quality indicators in my ADS-B Ground Station but we noticed that is not the same value ex :
                NIC aircraft:10

                NACp aircraft: 10

                NIC ground station : 8

                NACp GS : 11

And he tell me it should be the same value between them (aircraft, ground station)
Is there any explanation about this and what is the tolerance btw the transponder and GS ?

These are digitally encoded messages. If the ground station is displaying the data it decoded from the aircraft then either it is coded improperly or it was encoded improperly by the transmitter  or you’re not looking at what you think you’re looking at.

How does the pilot know what the ADS-B quality indicators were in the aircraft? Normally this information is not known to pilots.

@DeltaLima thanks for your support , this aircraft is for flight check , to Calibrate VOR,ILS,PSR,MSSR-MODES , so it's equiped with special .    Please can answer to this question # ADS-B MOPS & quality indicators I added before and no one answered to this question. Thanks again


text:
Does ADS-B use its own antenna?
In the aircraft we have:

ADS-B out/in using 1090ES
A/C or Mode S transponder for SSR grounding interrogation.

Does the ADS-B out use the same antenna as Mode S or does it use its own antenna?

The truth is ADS-B over mode S is not a stand-alone LRU, but a software in the transponder LRU. The only antennas are the antennas of the transponder.


text:
Can an EASA Part 66 AME License be accepted by the FAA?
I am holder of an EASA Part 66 AME License (Category L3H / hot air balloon). Can it be accepted by the FAA so that I would be allowed to do maintenance and annual inspection of American-registered aircraft?
I did not find any form/application on the FAA website to get it approved/accredited.


text:
Why does the EA-18G not have a gold-lined canopy like the Prowlers have?
I noticed on the Growlers that they have a clear canopy compared to the gold lined canopy of a Prowler, Which protects pilots from radiation. If the Growler doesn't have a Gold lined canopy that  does that mean the Growler produces less radiation that is much safer for pilots? Or is there some other reason why its clear?

Antenna design was well known by the time the prowler was made. Every electron hitting the cockpit is one that can't be used for jamming. I think a proper answer needs a well placed source since the only discussion about it online is a bunch of 2nd hand hanger talk.

Did you mean "by the time the Growler was made"?


text:
What is the purpose of pipes that are sticking out of the Boeing 747 FWD Cargo door?
I went to visit an Aviation museum recently and noticed these pipes that are sticking out of a Boeing 747 in the FWD Cargo bay...
What are these pipes called and what is the purpose of them?

This is not just 'a' 747, it's N7470, the 747 prototype on display at the Museum of Flight in Seattle. As such there may be any number of items fitted to the airframe for testing purposes. These pipes don't appear on production aircraft, so I expect that they're something to do with testing carried out only on the prototype.

Curious.  Based on the flythrough at the Boeing Museum website, they appear to be located on the starboard side, just forward and below the forward-most flight test water ballast barrel. So, how about drainage for those?


text:
Do EA-6B Electronic Countermeasure Officers focus on one task at a time?
I was wondering do the 3 Electronic Countermeasure Officers on the EA-6B all focus on doing the same task at once? Or does each of the ECO have a specific task assigned to them on the mission that they just focus on (Tactical jamming, Navigation, etc)?

Prowler ECMOs rotate seats, and while sitting in the front right seat they function as a copilot.  There is no second set of controls as some aircraft have, but they are responsible for operation of the navigational and communication equipment.
In the back seat their primary responsibility is to operate the tactical mission equipment.  This includes radar signal receiving, analyzing and jamming.
Actual sharing of these duties between the two backseaters was largely dependent on workload, crew experience, and personal preference.  There really weren’t any hard and fast rules other than to work as a team to maximize effectiveness.  The mission commander for each flight had a responsibility to get the job done, but otherwise had leeway in the leadership style and method used for assigning tasks.


text:
What are the white tubes underneath the Canopy of the F-22 Raptor?
I was looking at the Raptor and noticed the white looking tubes underneath the canopy. Do the tubes have a purpose?  If so what is the purpose/function of the tubes? Are there any other aircraft with this kind function? Thanks.

Speculating: this could be part of the canopy locking assembly, likely in addition to the six grey points you can see along the middle rim of the canopy.

Chemtrail storage containers

@Richard clearly its plastic downpipe from rainwater spouting, being used as air ducting so the incoming breeze is redirected at the windscreen to keep it defrosted....

"Do the tubes have a purpose?" While I'm not an airplane designer, I'd bet everything on a (modern fighter) plane has a purpose - otherwise it would be removed to reduce weight.

Those are Cartridge Actuation Devices (CADs) - pyrotechnic charges used during canopy separation for canopy jettison or ejection. Note the nozzles at the outboard ends and the ignition wires inboard.
You can see a ground test of a similar unit installed on an F-16 below.

What is that person doing there? Looks like they are holding something, a microphone or other sensor perhaps. Surely, given there are two rockets firing and in a few moments a canopy falling from the sky, they could have set up a tripod or something.

It might be the guy who pulled the cable triggering the pyrotechnic cartridges (?)

Looks like they're following the instructions on the side of most fighters, 1. Push button to open door 2. Pull ring/handle out X feet to jettison canopy. I feel like I'd want to tie on a longer rope and stand farther back if it's not an emergency, but maybe there's some reason they can't.

He's wearing a hard hat, and what looks like some sort of protective suit (hard to tell from this low-res image, but it looks like a fireman's coat, designed for protection in burning buildings and the like).  He's probably fine.  Probably.

@CarloFelicione from other photos in this sequence, and a video of what looks like the same test, it's a person yanking the cord. A lot more nonchalant about the canopy flying overhead than I'd be.

@CameronSS cool guys don't look at explosions

@CameronSS excellent!  I’ll use that as my reference!


text:
Why does lift coefficient not change with velocity in inviscid flow?
Why exactly is lift coefficient constant with velocity in inviscid flow?
(For a given angle of attack, 2D section and fluid, pre-stall etc)
Does it come out in the derivation of lift coefficient or does it just happen to be true?
The usual equation, Cl = Lift Force / (1/2 fluid-density X velocity-squared X area), just enables us to calculate the lift force knowing the velocity because Cl is constant with velocity
Thanks

We have a natural phenomenon, which is that an airfoil creates lift, and scientists, engineers and others have been studying it for several centuries and came up with different physical-mathematical models describing what is happening, with varying levels of accuracy. One of those models is that you can determine the amount of lift (a force) by multiplying dynamic pressure (proportional to velocity squared), the reference area and a "coefficient of lift", which represents the effects of shape of the wing/airfoil and anything else that's unaccounted for.
Now, in practice, experiments showed that this is an accurate enough model for reasonable speeds, which are low compared to the speed of sound (you only mention "inviscid", but not "incompressible", but I'm assuming it here), and high enough for viscous effects to be limited to a very thin boundary layer, and low enough angles of attack to have no detached flow. Turns out, coefficient of lift is basically constant for a given airfoil at a given angle of attack in the aforementioned conditions and with the definition given above.
"Inviscid flow", which of course does not exist in reality, is another one of those (idealized/simplified) models that people came up with to be able to predict the flow around airfoils and airplanes and all kinds of objects, because they realized that it is accurate enough at least for certain shapes and for certain flying conditions. From the way inviscid flow is defined and how the pressure distribution around an airfoil is determined, it turns out that the coefficient of lift will indeed be exactly constant for a given airfoil and angle of attack.

Thanks! Yes, incompressible - sorry I forgot to add that. IIRC wrt Inviscid - thats why the plots in Abbott and Van Doenhoff change slightly with Reynolds especially at higher AoA / separation. My question is more 'Philosophical' 'why' does it 'turn out' that Cl is const (assuming the above)? Is it possible to explain this reality in terms of mechanisms / regimes or other physical things?

Personally, I feel about that question the same way I feel about "Why do airplanes fly?": Not "because the air goes faster over the top something something Bernoulli" or "the air is deflected downward something something Newton", but simply "Because they do." Newton, Bernoulli etc. are just means to allow us to grasp the phenomenon better. Same with "Why is there gravity?" and other questions like that. If you want to ask "How ..." then the answer is indeed all the different models and explanations that we came up with.

Thanks. Yes, I see what you mean, our models are just models of reality.  I think I am overthinking :)

@ Raketenolli: Perhaps the answer to my question is that in our equation / model none of the stuff that isnt explicitly referred to in the equation but is lumped into Cl does not change (significantly) with velocity (given invisc, incomp. and attached)? Cl includes foil shape and angle of attack , which dont change with velocity for example?

Only you know what is an acceptable answer to your question ;-) But if it is inviscid and incompressible it cannot be a detached/stalled flow, and the $c_L$ will actually be perfectly constant for a given shape and angle of attack, not just "not change significantly".

I guess we should start with saying that over many orders of magnitude of Reynolds number, it would be likely that the coefficient of lift would in fact change. This is why insects, birds, and aircraft use different aerodynamics, depending on their size. However, if we are assuming the presupposition that it doesn't change is true, we can limit this to be for an aircraft in a certain range of viscosities.
Basically, the relative difference in the flow speed, how much faster the upper flow is relative to the lower flow (equivalent to the how much higher the lower pressure is relative to the upper pressure), is independent of the freestream velocity, and is dictated by the properties of the fluid and the geometry of the aerofoil. Even that is not necessary, it turns out that if an aerofoil is thin enough, then even the geometry does not affect the relative velocities or pressures.
So, while in a faster flow the total lift force may be greater, the coefficient of lift is only determined by the angle of attack, in a fluid with viscosity. That is, for a given freestream velocity (the TAS of an aircraft) the relative change in static and dynamic pressure is independent of the initial velocity and is only a function of the angle of attack.
Just to pre-empt any comments, circulation is a difference in flow speed above an aerofoil relative to below and is used to calculate coefficients of lift. It may be more common to talk about this as circulation, but when added to the freestream velocity component from potential flow, the net result is flow going much faster over the upper surface and much slower under the lower surface. And very importantly, when we do experiments in wind tunnels, we measure pressure using manometers so there is a lower pressure above relative to below. These two phenomena are in fact interchangeable, hence the reason we integrate the pressure coefficient along the upper and lower surfaces which is equivalent to the closed contour integral of circulation.


text:
How to find ATC recordings for a May 8th general aviation crash?
Unfortunately this Sunday I had a friend and her husband who crashed and didn't make it.
Would it be ridiculous to ask if anybody perhaps would know where to look for potential ATC recordings? I have found archives such as LiveATC.net, but I don't even know where to begin for which towers they hopefully communicated with before putting it into the dirt.
It appears as though they had loss-of-power if you look at the flight data, and vectored to land on a nearby freeway.
I'm curious if they may have reported their situation to Center so we can help all of their friends (big aviation community around them) get some understanding and closure. Yes, there will be an investigation report in 6-12 months, but obviously we would be curious to gather what we can for now.
I will link the ADSB data and perhaps some folks in this community would be able to suggest which towers they would be communicating with so I can better refine my search in the audio archives.
The end of the data in the link is the last reported location of their flight, so that should help us get an idea of which towers they were in communication with and at approximately what time.
Any help is greatly appreciated!
Links:

flightaware.com
aviation-safety.net

I'm very sorry for your loss. It's not a ridiculous question at all, and I understand your motivation. Before you go down this route you should consider the personal impact of listening to a recording of that nature.

Also if we find these recordings, everybody in the world will be able to listen to them as well.

I have found one audio file which contains communications from N801DT near the beginning of the flight. I was not able to find anything more than that.

BVU does not have an airport traffic control tower; instead, pilots self-announce on a common traffic advisory frequency. That frequency (122.700) is not recorded by LiveATC and likely wouldn't tell us much if it were.
BVU is under the jurisdiction of Las Vegas TRACON. The frequency providing services in the immediate vicinity of the airport is 124.475, which is not recorded at LiveATC. At their altitude, the pilot would not have been required to be in contact with ATC.
As the aircraft climbed out of BVU and flew northbound, they entered the Las Vegas Class Bravo airspace. They were required to establish communications with Las Vegas TRACON prior to entering this airspace. They did this on the frequency 119.400. You may hear their initial communications beginning at the 28:20 mark of the KLAS4-App-NE-May-08-2022-1700Z.mp3 file accessible from the LiveATC archives, that is, the ~30-minute audio file which begins at 1700 UTC on May 8th. (The time referenced is elapsed time on the downloaded file. The file as played in-browser may have slightly different times.)
The pilot is instructed to enter a specific squawk code and is identified on radar.
The pilot requested flight following, meaning that even after they departed the Class Bravo airspace and were no longer required to communicate with ATC, they wanted to remain in communication and receive traffic advisories, etc. This bodes well for your goal to retrieve audio archives for their entire flight.
Unfortunately the pilot was instructed to contact Nellis Approach Control on 135.100, and that frequency is not recorded at LiveATC. Nor are some of the other likely frequencies for their route:

Nellis Approach 126.650
Salt Lake Center 133.450
Salt Lake Center 132.250
Salt Lake Center 118.050

Los Angeles Center 124.200 was recorded, but I listened to an hour of audio and it seems that N801DT did not enter that controller's airspace.

You can rest assured that the FAA is compiling the audio from this flight and preparing transcripts which may be released to the NTSB. I believe it is the policy of the NTSB to not release audio files, but you might have luck submitting a FOIA request to the FAA directly. I suspect they also might not release this data, though, particularly while the investigation is still underway. (See Chapter 3, Paragraph 9 of Order 8120.11 and Chapter 14, Paragraph 3 of Order JO 8020.16.)

Wow. Tremendous thank you Randomhead. I can't tell you how much I appreciate the time and effort you put into helping.


text:
What's the difference between itinerary and segment?
I know that a segment is defined as the full flight from the airport where passenger A initially  boarded on the airplane to his/her last destination. A segment could be composed of multiple legs. However I don't understand what's the difference between a segment and an itinerary?

An itinerary often contains multiple segments. Consider a round-trip booking.


text:
Is there a good resource for ACARS messages?
Is there a good place to find out all the types of data that ACARS can send and receive? I've seen high level descriptions, but I'm looking for the data that actually gets fed from or to the Flight Management System.

I believe that this would be airplane specific and may even be proprietary by the manufacturer.  You might be hard pressed to find a good source.  ACARS itself basically just supports generic text messages.  How ACARS is actually used by the FMS can be a little more complicated.

Unfortunately the term ACARS is one of the most misunderstood and misused terms in aviation. ACARS - Aircraft Communications Addressing and Reporting System - was created as an end-to-end datalink between aircraft and the airline dispatchers. It provided basic text data using the ARINC Specification 618-8. (Unfortunately not free.)
As datalink functionality has grown, ACARS has been modified and patched and added to to support these new functions. What is defined in A618 is now classified in the datalink world as AOC - Airline Operational Communications. AOC messages can be found in ARINC 633-3.
The issue with that is that it is communicating with the dispatcher not ATC. That falls under the title ATS - Air Traffic Services. Early versions of this will be found in ARINC 623-3. That has mostly been replaced by newer applications defined in ARINC 622-5 which supports bit-oriented protocol (over the older character based protocols.)
In the last 20 years or so, there has been a plan to provide ATS globally. The European industry which wasn't tied as closely to the ARINC systems as the US took a more more clean sheet approach referred to as ATN - Aircraft Telecommunications Network which isn't interoperable with the ARINC standards. So the industry now supports a mix of infrastructure. As it has moved towards global capability, the standards are migrating into the ICAO ecosystem.
So the best reference available to find answers to almost any aviation datalink related question is the Global Operational Data Link Document (GOLD) published by ICAO.


text:
What is the altitude required to bypass Russia's airspace restrictions for airlines?
Given that Russia charges a lot for flights over Siberia from Europe to Asia, would flying above the
Kármán line (100km)
effectively bypass this restriction (from legal perspective) ?
If not, what is that altitude ?

The Kármán line is considered to be the upper limit of airspace and the lower limit of outer space. According to the Outer Space Treaty of 1967 outer space cannot be appropriated by nations (nations cannot claim ownership of parts of outer space).
In that sense, one would be able to fully legally avoid navigation (and other) fees and bureaucracy imposed by any country one was to fly over if it was done above the Kármán line, but since the stunt would require a spacecraft, it hardly would pay off...

Any aircraft that must generate net downlift in order to not gain altitude, can be safely considered to be orbiting rather than flying.

You are correct @ymb1 FAI and space trieties use Kármán line, while for example US consideres space to begin 20km lower. I might have to improve the wording of my answer. What I was after is that Kármán line is the "safe" option with not much of anyone claiming you to be in their airspace. For cost, the aforementioned 20k difference hardly makes, well, a difference.


text:
takeoff and landing distance GRAPH for a Cessna 150
I cannot find a takeoff and landing distance GRAPH for a Cessna 150. The C172 is easy to come by but why no C150?

Cessna publishes its takeoff and landings distances as tables, not as graphs. Example from a Cessna 150M:

If you find a graph, it's most likely some third-party has made it. If you want one for a 150 and nobody has made one, you'll need to make it yourself or ask somebody else to do it.

Thanks Chris. Yeah, the third party graph is what I'm looking for. I would have assumed that somebody would have made one of these for the (almost) most popular plane on the planet. I must be asking the wrong question.

@Greg Do note that different models of 172s have different performance- you shouldn't be using a graph for flight planning purposes unless you've confirmed it matches the one in your POH. Personally I find the graphs unwieldy anyway- using a table is much faster.


text:
What are the prerequisites to join a flight School ? Is it different for each country?
(I didn't finish Highschool, failed French and math fml)
I've finished my 3 year diploma in Architecture.
I'm 23. Nobody has ever supported me and my dream to be a pilot. Not even my parents, heck they even found it funny, that I could even dream that.
But I'm not gonna stop till I drop.
I just wanna know if flight schools need a student to be a highschool graduate. Cause I've only got GCSEs no A levels.
  I have 6 GCSEs with physics chemistry and math being 3 of them, and C grade on most. Does that help?

What kind of pilot do you want to be?

No highschool needed.  No degree of any kind.  Just money.  Now, an employer may want to see a diploma or degree.  I never cared if an applicant had one though (charter, not airline).

You may add your current state of research (have you already contacted schools? Do you know names of some schools? Any other relevant elements)

What makes you think you can manage the exams for flight school if you can't even pass a high school exam?
They're at at least the same level of complexity...
But no, in general you don't need a minimum education level to start flight training. That said, if you want to get hired as a commercial pilot at some point, many employers WILL have education requirements without which they won't give you a job.
This is for EASA requirements, which are in use in the UK where you seem to be located. And of course individual flight schools may well decide to not take on people they expect to not have the capability to pass the exams, as having a high rate of failure on exams may reflect poorly on them. But that's not a legal requirement and would be a policy of the specific company in question.

It should also be noted that professional pilot training isn't "take it till you make it",  failures during training can be grounds for being rejected from any future jobs, or may restrict the applicant to cargo ops or small aircraft.


text:
What's this prominience in the B737?
I have recovered some spotting pictures that I made long ago and I came to realize an upper prominence in the fuselage of this Norwegian Boeing 737. I don't remember to have seen it in other 737s.
Does anyone know what is it and its purpose?

What is the lump on top of the Boeing 737? Indeed, this one asks for the same feature. Thanks!

It's a SATCOM (Satellite COMmunication) antenna. It has several purposes, one of them is internet connection for the passengers.
There are different types and configurations. On some aircraft the antenna is in the front, on some it's in the back, some have two.
(image from http://www.b737.org.uk/latest-2016-04-06.htm)

I think that article refers only to specific a application. Most B737s don’t need Satcom communication as they are almost always line of sight with the ground. Most of those large antennas on the roof are for WiFi only. Most overseas aircraft has Satcom long before these WiFi antennas were installed.


text:
Where can I find whether or not an aircraft requires two pilots?
I am trying to build custom scenarios for myself and I wanted to know where is an exact place for me to look If an aircraft needs two pilots.


text:
Should I add or subtract PA when calculating performance
I am planning a cross country and I wanted to know how would I calculate my PA for performance.
If the elevation of my airport is 16ft
and the altimeter setting for the day is 30.03
I subtract
30.03
-29.92

.11
.11*1000=110ft
Do I add the factor to Field elevation? or would I subtract?
and what determines if I add or subtract?
And if I am climbing to an altitude of 6000, would I use a Altitude OF 6000?
2.Should I add/subtract that PA factor TO to or from 6000 to get my PA?

Alway subtract current altimeter setting from 29.92.  29.92 - 30.03 = -.11 * 1000 = -110 ft.  If the pressure is lower, the subtraction would be positive, and you add the result to field elevation.

@wbeard52 Ok I got that but what I am trying to understand is why would we subtract the conversion factor or add it if it needed to be added. Just because its a negative number we subtract and add if its positive? whats the reasoning behind that theory?

If the altimeter setting is higher than 29.92 it would indicate that a given volume of air is at a lower altitude. so when we apply the formula above, we get a negative number so that the pressure altitude becomes lower than field elevation.

30.03 is larger than 29.92. Pressure gets lower as your altitude gets higher, so a larger pressure corresponds to a lower altitude.
So, since the pressure is lower than standard, your pressure altitude is lower than your indicated altitude, and you should subtract this factor. Your pressure altitude on the field is -94 feet.
If you subtract the altimeter setting this way every time, you can always subtract. This is because you'll get a negative result when the altimeter setting is lower than standard, and subtracting a negative number is the same as adding.
When you climb to 6000 feet, that is an indicated altitude telling you how far over the datum plane of 30.03 inHg you are. To convert that to a pressure altitude you will want to subtract the same 110 feet, so the pressure altitude will be 5890 feet.


text:
Logging time as a student
Lets say I am a student with no student certificate but I have a good friend who has flown for years on an airbus and I want him to teach me how to fly. Can I log flight time being that I don't have a student Certificate?


text:
How should a Minimum Equipment List (MEL) be used?
I want to make sure that I understand how an MEL is used.
Does this list have instruments that are needed or not needed to conduct safe operations?
For example, if I have an altimeter that’s inoperative, but it is a part of the MEL, does that mean I cannot operate that aircraft?

For an airplane to be airworthy, it must comply with its type certificate.
For smaller airplanes, there is something called an equipment list and is typically found in the POH.  This equipment list (not the MEL) will show all equipment that was installed on the aircraft when it was certificated.  All the -R (required) equipment must be installed while -A (additional) or -O (optional) equipment does not have to be installed.
If any of the required equipment is not working or broken, the aircraft is no longer airworthy.  It would not be ideal to ground an airplane due to a burned-out light bulb, so the concept of the MEL (minimum equipment list) was created.
The MEL will list all items that can be inoperative for flight.  As @757toga mentioned in his answer, you must comply with any conditions on the MEL for the inoperative equipment to be airworthy for flight.
I personally think the name minimum equipment list adds to the confusion of what the documents' purpose is.  It is the maximum equipment that can be inoperative for flight and still be airworthy provided the conditions are met.

Equipment List: Will list all equipment on the airplane and mark as either required, additional or optional.
Minimum Equipment List:  Will list all equipment that can be inoperative for  flight provided the conditions are met.
Kind of Operation Equipment List:  Equipment required for certain operations (i.e. day or night, VFR or IFR, etc.)


text:
Pilot briefing strip
Hello can you guys break this down for me?
The briefing strip says
Rwy 10 R helicopter visibility reduction below RVR 4000 NA. Is NA not authorized or Not available?
Simultaneous approach authorized .
Inop table does not apply to S-ILS 10R

NA = not authorized. N/A = not applicable. It’s on the abbreviations page of the Terminal Procedures Publication (TPP).

This was ILS or LOC RWY 10L FLL

That’s a “Not Authorized” ie a RVR may not be below 4000 for helicopters flying the approach.
Simultaneous approaches authorized means that instrument approaches into runway 10L, 10C or additional parallel runways, are authorized by ATC to be flown simultaneously with the S-ILS 10R approach.
The inoperative table, i.e. the approach minimums with inoperative airport equipment are not applicable when flying an S-ILS approach.  Most likely, these are applicable to situations when one is flying a LOC approach to localizer only minimums.

Simultaneous approach authorized

Means that ATC can have aircraft flying approaches to both 10L and 10R at the same time.  ATC will have requirements on what they have to maintain as far as separation between aircraft, and those requirements can vary based on things like runway separation and radar monitoring.  For the pilot, we aren't given their whole rulebook; we're just told that they can run approaches to both runways at the same time.
If the runways were too close together, then you wouldn't have this notation and the in-trail spacing would be the same as if all aircraft were flying an approach to the same runway.
In some cases like Atlanta, where you have 8L (usually arrivals), 8R (usually departures), 9L (usually departures), 9R (usually arrivals), and 10 (both), the chart may say something like, for 9R, "Simultaneous Approach authorized 8L, 8R, and 10", meaning that they can have an approach to one of the 8's, and 9R, and 10 all at once, but not to 9R and 9L at once, since they're pretty close.
That notation doesn't spell out all the restrictions that ATC applies, but it tells you that it's okay to be on this approach and see (on TCAS) aircraft on a parallel approach with less than normal (3 NM) nose-to-tail separation.


text:
Advice on establishing a solid career
1.Do you feel that students who does not have the opportunity to attend the popular schools can be at a disadvantage when applying for jobs at the airlines?
Rant
My Thing is I would love to go to a school known for their outstanding programs and thats not to say those schools are/are not pushing great pilots but cost wise, I just don't have it and the time to save for it, is not an option.
Second question is, How does one groom themselves to become that of an airline pilot.

To get hired at the airlines now (and for the foreseeable future), you need the necessary ratings, and you need experience (hours).
When you have those, nobody cares which school you went to.
There have been times when airlines had multiple applicants for every opening, and in those days, having an "in", either from having gone to the right school or being part of the right network or knowing the right people at the airline, was tremendously helpful in distinguishing yourself from all the other candidates. That isn't the case now. If you meet all the requirements, don't have any grave red flags (i.e. DUI or other criminal history) on your resume, and don't say &$#@ too often in the interview, your chances of getting hired are very good.


text:
DA40 avgas leakage on filling
New owner of a 2012 DA40 50 gal fuel capacity.(Also new to aviation!)  Recently was having the right wing filled from about 14 gal and lineman stopped within 4 gal when he noted a puddle on ground. not clear if came out of fuel vent.  Mechanic checked over and found normal function. later completely filled and no problem.
I have been unable to find any diagrams to show how the 3 tanks in the wing are connected or the mechanism for venting.  Hoping for answers and education. Thanks.

welcome to AV.SE. I'm sorry, I don't understand. what is your question?

I don't know the reason it dumps out the fuel vent, but I can tell you from my experience instructing in DA40s for several years that you can top-off the 40 gallon tanks without them dumping out the vents, but with the 50 gallon tanks they will dump out the vents if you fuel to the caps.
I always asked the fuelers to "leave it 2 or 3 inches below" the top/cap to avoid this.


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Why does the MVA have 1000 ft height buffer?
As per definition the Minimum Vectoring Altitude (MVA)

in each sector provides 1000 ft above the highest obstruction [...].

I am curious where these 1000 ft stem from. Why not 500 ft? Why not 300 ft, or 600?

Related: Is there an ICAO document that describes MVA criteria?


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PPL EASA(A) exam cross-country flight altitude
I need find explanation for EASA PPL(A) question. According to the Boeing Courseware correct answer is "a", but I can't find explanations in course materials neither in SERA or German AIP (questions are Germany specific).
Provided that no other regulations or requirements demand a lower or higher altitude, the Minimum Safe Altitude for cross-country flights with powered aircraft is at least:
a) 600 m AMSL
b) 150 m above ground or water
c) 300 m above ground or water
d) 600 m above ground or water

From my perspective cross-country flight is keyword there, and I can't find clear definition other than "flight in vicinity of aerodrome where a pilot don't need to observe traffic pattern traffic".
Please advice.

Did you check the SERA for minimum altitude or heights? What does it say and how does that not answer your question?


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Under EASA regulations can I log PIC hours on exercises for a rating already completed?
The scenario is this: a zero-to-hero modular CPL program is contracted at School A. Due to delays, the student decides to jump ship and complete the CPL exercises at School B in order to enable him to work on commercial ops before his training is finished at School A.
After receiving the CPL, the student continues training at School A in his free time, since he has already paid for the training.
The question is this: When he gets to the CPL exercises, what are the rules for logging time? In principle, can they be logged as PIC time, since the student is already in possession of a CPL, even though from School A’s perspective and according to their syllabus, the exercises must be flown as Dual?
To clarify, this is training done in Europe, under EASA rules.

If you don't need the dual training for a rating why wouldn't you apply the pre-paid credits towards flight time instead of paying for a CFI you don't need?

@Michael Hall, because the school refuses to modify anything. They won’t even consider ratings completed at other ATOs within the context of their syllabus

My reading is that this is between you and the school.
Part-FCL describes the rules for logging PIC time in FCL.050(b)(1):

(i) the holder of a licence may log as PIC time all of the flight time
during which he or she is the PIC;

It's between you and the instructor who the PIC is.

(ii) the applicant for or the holder of a pilot licence may log as PIC time all solo flight time, flight time as SPIC and flight time under
supervision provided that such SPIC time and flight time under
supervision are countersigned by the instructor;

Student Pilot in Command (SPIC) is defined as "a student pilot acting as pilot-in-command on a flight with
an instructor where the latter will only observe the student pilot and shall not influence or control
the flight of the aircraft." If the instructor is content just to observe, you are acting as SPIC and can log PIC time (though the instructor needs to agree to countersign it).
The next four paragraphs are not relevant to your situation.
So you can log the time if the school consents to designate you as PIC (unlikely) or the instructor agrees to sign off on SPIC time.
On the other hand, I've heard anecdotally that different competent authorities have different ideas about what should be logged. It's worth checking with your own before logging time you're not sure about.


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why normal surface pressure over wedged cone is greater than freestream pressure
I came across this question in "Fundamentals of Aerodynamics" by John Anderson, although I knew how to solve the question, I didn't understand why we had the surface pressure on the wedge 1.31×10^5 N/m^2 higher than the freestream pressure 1.01×10^5 N/m^2, I thought it is intuitive to consider the freestream pressure always higher than surface pressure.

Here is the question if you needed more information

There is a difference between total pressure and static pressure. The total pressure is always higher in the free stream, while the static pressure can be higher at the surface. A simple idea is to think about it like total energy (total pressure), which consists of the the kinetic part (dynamic pressure) and the potential part (static pressure). While you are losing energy because of friction you can still convert kinetic Energy (dynamic pressure) into potential Energy (static pressure).

At supersonic speed, air makes way for a body by increasing density and pressure, so the same air takes up less volume when flowing around this body. It does so by forming compression shocks when the inclination of the body surface to its direction of movement increases, and by forming expansion fans when that inclination is reduced or the body contracts again. A second shock is produced when the flow around both sides meets again at the end of the body.
That free stream pressure is higher than surface pressure is mostly true only at subsonic speed, where air speeds up (and reduces static for an increase in dynamic pressure) to make way for a passing body, except at and near the stagnation points (rsp. stagnation lines on wing-like structures).

Pressure at the surface facing the free stream is higher than free stream pressure, for supersonic as well as subsonic flow. The body surface is displacing a certain volume of air - at subsonic speeds the body can warn the air that it is arriving, slowing it down in exchange for a rise in pressure. With the highest pressure at the stagnation point.
Image source
At supersonic speeds there is no fore-warning, air can only be compressed after the stream hits the body front. Nevertheless, compression still takes place because of the body taking up air space.


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What is the windshield Heating Element (Thin Cable) for Aircraft or Rotorcraft?
My question is related with the heated windshield employing the some type of heating elements. Very very thin metalic cables are in use between two glasses of windshield. I have searched but noting found about its material. Do you know any cable supplier provides/gives the below graph. I need a wire to use as a temperature sensor. The needed TCR coefficient temperature in °C is 0.0048-0.0050. Diameter can be between 0.02 – 0.03 mm. Would you have any suggestion to us about material / alloy?

Are you seeking a generic wire of these properties for a non aviation application, or is this to repair an aircraft? If you are after the part the electronics stack exchange may give you a better answer.

You can get generic, thin film style heating elements ready to go from various suppliers. If you are replacing an existing windshield on a current aircraft you will need to source a proper, certified, windshield for that airframe which should have the wire already embedded in it.
It seems that at least some of the elements are made of NiChrome


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Constant Speed Propeller
Dear All I know that this topic is not new, but I really tried to grasp the concept using available sources (which are actually pretty much the same). So, if possible (really appreciated):

I'd ask for an practival example comparing two aircarfts (one with Constant Speed Propeller and another without) in the same conditions showing the advantage of the first one. Something like "you see with this one (without) our hands are tied and we can't do anything so we are buring fuel, wearing enging, etc, and with this one (with) we can just adjust and ... ".

I'd ask to clarify why fine pitch is considered more efficient (e.g. during the take - off). I can't understand why we increase AOA of the wing in order to produce more Lift and in regards of Propeller (which is basically the airfoil we consider low pitch as MAX performance)

last one... there are a lot of explanations where I see "and now propeller increase pitch and slow down..." Are we trying to limit engine output in a way of applying some resitance to the engine? If yes, why?

Sorry for potentially stupid questions, but I really can't get this one.
Thank you!

I don't know if you've already checked our existing questions on constant speed props, some of them might be helpful, e.g. this one on blade pitch during takeoff. Also please note that we strongly prefer to have only one question at a time; the [tour] might be useful if you're new to StackExchange.

You are correct that AoA is very important for propeller (airfoil) to be efficient.
The only reason to change pitch when going faster is to account for the change in relative wind because the aircraft moves forward faster and faster while propeller spins at a constant rate.
So, we have fine pitch for take-off run and climb (slower), then adjust pitch to coarse for cruise (faster).
We always try for best airfoil AoA to be most efficient based on prop rpm and airspeed as illustrated:

Thank you. Any chance there is a picture which will visualize this? Somehow. I mean this relative wind component and it's direction.

@Sergey see added picture

Thank you, trying to get my head around it.

Pushing a prop into fine pitch is like downshifting your car's transmission. In both cases, the engine is allowed to spin up to its maximum speed and thereby develop maximum power- for taking off or for passing a slow-moving truck.
Pulling the prop back into full deep pitch is like shifting your car into its top gear. In both cases, the engine speed falls which reduces the amount of power it can make but increases fuel mileage and decreases engine wear. This is best for constant-speed cruise under the most economical conditions.
And the prop pitch can be adjusted to any desired value between the engine's redline and idle RPM, almost like a 5-speed car transmission. "Mixing up the gears" lets you choose the best combination of power and economy for any flight regime.

I was going to offer the same comparison.  Thing is though, depressingly few people can even relate to shifting gears any more.  Maybe on a bicycle?

@MichaelHall, what an amazing world we live in. I've driven 3-on-the-tree, 4-on-the-tree, 3-on-the-floor, 4-on-the-floor and 5-on-the-floor... oh yes and I knowhow to use a rotary phone!

This answer takes a different look at the constant speed propeller.
I was confused about them too, many years ago.
I think the confusion started because everyone always describes the prop lever as controlling the pitch, and full forward as decreasing the pitch. Leading to the assumption that full forward must be least pitch or flat. And as we all know a "flat" blade (i.e., zero angle of attack) produces no thrust. So why do you want no thrust for takeoff?!? But this is not the way it works (or at least , not the way to think about it)
Let's start with the name:  constant speed propeller.
As the name implies a CS propeller tries to maintain a set RPM in the face of changes in engine power settings.
Think of the prop as always trying to absorb all the power the engine is putting out and thus reach equilibrium.
The prop lever sets the prop RPM or really, the upper speed limit of the prop.
Full forward says, "Go as fast as possible".
Of course, for a given power setting, the prop can only go faster by decreasing the blade pitch.
At any given prop setting, the prop is basically modulating its pitch in order to maintain the selected RPM.  If it's going too slow, it reduces pitch which allows it to speed up. If it's going too fast, it increases pitch which forces it to slow down.
Essentially, the constant speed prop always tries to absorb the engine's power at the speed selected by the pilot.
For takeoff one wants the most thrust you can get and since the blade is just an airfoil and the thrust for the propeller is really the lift generated by the blade's airfoil, thrust (1/2 ρ  V2 CLα  α  A) goes up more quickly with the relative wind velocity (V) than it does with angle of attack (α) so we would like the highest V (RPM) that we can get.
But here's where the confusion comes in.  When you set the Prop to fine pitch it doesn't go to "flat" and stay there.  What you are really doing is saying go to "full fast". Once the blade reaches its top speed allowed by the prop governor it will try to maintain that speed by increasing the blade pitch in order to absorb the rest of the power put out by the engine.  Therefore, in this takeoff configuration you're developing maximum engine power and running it through the prop at it's maximum speed, thus developing maximum thrust.
Now, what happens when you reduce the prop while maintaining full engine power?  Well, the prop sees it has to slow down, so it increases blade pitch. This increases drag on the prop which is seen as torque at the engine.  With that increased load on the engine the engine slows down. (Think about going up a hill in a car with out adding pressing a bit more on the accelerator)  But note that load on the engine really transfers back all the way to the cylinders and pistons so that the pistons see an increase in resistance to motion.  Inside the cylinders this  "push back" from the pistons results in higher pressures being developed in the cylinders and higher torques in the drive shaft.  Too much torque in the engine and it breaks.  Too much compression in the cylinders and it starts acting like a diesel and detonates prematurely.
So,clearly, pushing too much power through the engine/prop is bad!!
To prevent this you always need the prop set so as to be able to absorb the anticipated new engine power setting.  So when moving to a higher power setting adjust the prop first so it is allowed to speed up before adding power.  And when moving to a lower power setting, always reduce the power first before slowing the prop (i.e., increasing blade pitch/torque).
Also note that if the engine is not producing enough power, the prop attempt to go to its lowest pitch- trying to get the RPM up, but it won't be able to.  This is what happens when you've got the prop lever full forward but the throttle at idle.
As far as why a CS prop is better than a fixed pitch prop, it comes down to being able to control the angle of attack of the propeller blades.
We know if you set too low of an RPM at a given power setting the prop will try to absorb that extra power by increasing pitch.  It will do this without regard to how efficiently the prop is operating. That is, it would take the blade all the way to 90 degree angle of attack if it could to try to slow the prop down. Obviously this isn't desirable since it would not be generating any lift/thrust - it would all just be drag.  You can get out of this inefficient condition by setting a higher prop speed, causing the prop to decrease blade angle of attack in order to reach the higher RPM, or by reducing power also causing the prop to decrease blade angle of attack in order to maintain the set RPM.
At some point in its travel from full blade pitch to no blade pitch the prop will cross a point of "optimal" blade pitch for the given flight conditions.
Therefore for a given set of flight conditions there is an optimal power/speed combination that allows the prop to operate at its optimal angle of attack.
For best performance you always want to make sure you demand an RPM that "matches" a given power setting so that you keep the prop operating at its optimum efficiency.  The more useful and important of these combinations are given in the aircraft's POH.
Other answers here discuss how the angle of relative wind as seen by the prop changes with RPM and forward airspeed so I won't repeat that. But the thing to remember is that unless you can change the blade angle, then for any RPM there is only one forward airspeed that delivers optimal blade efficiency.  if you're not there then you are getting more drag than you need to for the given thrust generated.  Some props are designed to give best performance in climb ( a slower airspeed), some at cruise ( a higher airspeed) and some a compromise somewhere in the middle.  They still work, and are less expensive to maintain and are easier to operate but they aren't as efficient.
Bottom line:  Don't think about setting a blade pitch, 'cuz that isn't what's happening. Think about setting the RPM at which you want the prop to absorb the engine's power.
Hope that helps.

Thank you a lot for detailed answer. You got the root of confusion very precise (I mean "flat on take off" especially). I think I need to read it again to grasp all details, but for now could I clarify one thing - if the CSProp is able to find an optimum - why do we have the level to set a limit for it? Would it be enough just to ajust power (throttle) and to achieve equilibrium by govener work? Why do we need the possibility to say "don't go this fast" using the language above (it helped by the way). Also, relative wind here (just to check if I got it right) is what blade hit, not aircraft?

@Sergey - It can’t find an optimum by itself, it only provides the means by which the pilot can set it if he knows it.

Thank you, and one more question, if possible - one question from exam samples  "Which statement regarding the "constant-speed propeller" is correct?" has the right answer "The pitch of the propeller rises with higher speeds." Is it aircraft speed or blade? Because if it's a blade - I don't understand the tendency "high RPM - lower pitch and vice versa". THanks again.

Sergey - That question (with the context you've provided here) is not a very good question IMO.  But if they say that's the right answer then they must be talking about a/c speed.  (See the drawings in @Roberts answer) Note that to achieve the same AoA the blade pitch must be higher when the prop RPM is held constant but the a/c speed is increased.  But while it's possible to work backwards to justify this answer because we know what it's supposed to be. It wouldn't be possible to know whether they were talking about blade speed or a/c speed in the exam. Unless the other answers were false.

@Sergey note that the propeller governor on the engine has no idea what the propeller blade angle is. It just knows that when it sends oil this way, governor speed goes up, and that way, governor speed goes down, and it wants speed to stabilize at some value based on spring tension on internal fly weights.  What blade angle results from what RPM, manifold pressure and airspeed condition is a function of the propeller's design and how it is all matched to the engine.


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The Effect of Pitching Moment on Aircraft Stability
I'm designing a small RC plane and I'm currently at the first iteration of airfoil selection. I have converged to Clark-Y, NACA-2412, NACA-2415, and S-8036.
In comparing the four, I noticed that all of them have a negative Cm up to the critical angle of attack. Clark-Y's Cm values, however, are consistently more negative for all angles of attack, compared to the other three. I was wondering if this can be considered as an advantage for Clark-Y as less downward pitch would be required from the horizontal stabilizer. Could this be a disadvantage at level flight since it can cause the plane to nosedive? I noticed that at AoA = 0, Cl = 0.4 which could stop the nose from pitching down?
I have included a picture of Clark-Y aerodynamic graphs for your reference.
Thank you in advance for your help

Bear in mind that you probably don’t have the capability to reproduce that airfoil section to the level of precision required to do anything more than approximate those graphs. So the small differences in performance characteristics will likely be lost.

I’m voting to close this question because it belongs on Drones.SE.

@RalphJ -- we have a "model aircraft" tag here.

@quietflyer Anybody can create a tag, and Drones.SE hasn't always been around. It is now, and it's a far better fit for this question.

@RalphJ -- but is it really?  Have questions about airfoil design, pitching moment, and stability gotten a good treatment on Drone SE?  A search through ASE Meta suggests the issue of model airplane or drone postings is unresolved -- https://aviation.meta.stackexchange.com/questions/4072/can-i-discuss-drones-and-uavs-in-aviation-stack-exchange -- I support this answer -- https://aviation.meta.stackexchange.com/a/4085/34686 -- it might be time for a new Meta discussion --

Camber in the main wing airfoil means that the wing's center of pressure will move forward with increasing angle of attack (AoA). Since the reference point for the pitching moment coefficient c$_m$ is the airfoil's quarter point, this drives the c$_m$ to negative values. By choosing the quarter point as the reference point, the value of c$_m$ will ideally stay constant over the range of AoAs with fully attached flow.
This movement of the center of pressure destabilizes the airplane slightly and in consequence requires a slightly larger tail surface in order to trim the airplane over its useful AoA range. But it will only make the airplane nosedive when the tail is missing - the large lever arm of the horizontal tail gives it a strong stabilizing effect.
Now it might seem that a large, negative c$_m$ causes more drag from the larger tail surface when you compare the same design with an uncambered wing airfoil, but camber will also allow the wing to reach a higher maximum lift coefficient. For the same minimum speed, the cambered wing can be made smaller and lighter, so in total camber reduces overall drag.
The Clark Y is popular because it has a flat lower side which makes building the wing simple if you use traditional construction methods. It is also, together with a few other airfoils like the Göttingen 398 and 449, the basis of the thickness distribution of the NACA four digit range. However, airfoils designed with a desired pressure distribution in mind (as opposed to the pure geometry) will yield better results. Maybe you should look into Eppler or Quabeck airfoils instead which have been designed with the use in model airplanes in mind.


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How many coaxial rotors could you theoretically stack on top of a helicopter before the design becomes infeasible?
All coaxial-rotor helicopters in existence today have only 2 counter rotating rotor blades. An example of such a helicopter is the Kamov Ka-50. Coaxial-rotor helicopters have one big advantage compared to single-rotor helicopters and that is that they can lift a lot more.
My question is: Has anyone ever tried putting 4 counter-rotating rotors on a helicopter in order to increase its lift capacity even more? Would placing 4 or more stacked rotors on top of a helicopter be feasible?

"they can lift a lot more": The Mi-26 with a MTOW of 56 tons can lift a 20-ton workload with a single rotor, and many heavy lift helicopters don't have coaxial rotors, Kamov seems to be the exception.

Coaxial rotors don't have an advantage in lifting power because they have more rotors. The main advantage in that respect is that they don't need a tail rotor to maintain yaw control, meaning all the engine power can drive the main rotors.
Putting two rotors on the same axis increases the complexity quite a bit, but lets you remove the tail rotor. Putting four rotors on the same axis increases the complexity a lot more, for basically no extra benefit.
But it has been tried. Kind of. If you count a rotor made of biplane wings as two rotors, anyway.

Coaxial-rotor helicopters have one big advantage compared to single-rotor helicopters and that is that they can lift a lot more

Coaxial rotor helicopter do not lift more than conventional helicopters: the same 7,700 kg (16,976 lb) of the Kamov Ka-50 can be easily lifted by the single rotor of the H175 for example.
And as normally wrongly thought, coaxial helicopters do not even have the advantage that, missing the tail rotor, all the engine power goes in the main rotors: aerodynamic interferences between the two rotors make the lower one loose some 10 to 15% efficiency; and since in a conventional helicopter the tail rotor eats some 10 to 15% of the power available, let's just call it even.

Coaxial helicopters have only one advantage in respect to conventional helicopters: compactness.
Indeed, stacking two rotor one above the other, means that:

the tail rotor with its tailboom is no more required and;
the rotor diameter can be reduced, being the  thrust the same.

Being more compact a higher manoeuvrability can be expected which is for sure important for military helicopters.
Anyway this brings some disadvantages:

a horizontal tailplane (and the relevant tailboom) is still needed for stability, so you don't really get rid of the whole tailboom;
diameter of the rotor cannot be reduced too much before running into performance degradations;
as said, aerodynamic interferences between the rotors make the lower rotor loose some 10 to 15% efficiency;
rotor head becomes packed with rods and other rotating mechanisms increasing (a lot) the total aerodynamic drag and mechanical complexity (picture source);

so many exposed mechanical part might fail and are for sure a target in military operations;
the second rotor atop shifts the CG upward which might reduce stability.
and for sure I'm forgetting something.

For all these reasons a coaxial design is really important where compactness is at a premium. Packing more rotors one on top of the other will only worsen everything, especially points 3., 4. and 5., without bringing any improvement.

That's not the only advantage. Maneuverability characteristics are significantly different, and mostly better (but there are some areas where a coaxial is also restricted).

@Therac: well yes, compactness reflect also in manoeuvrability, I'll update my answer


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Can an airliner with no brakes be slowed down after landing by steering it into the grass?
If pilots have no control over brakes / air brakes / reversers, can they slow down the plane after landing by carefully steering it off the runway into the grass? Would it be better to put only one gear into the grass or all three?

How could that not be true? Whether it's a good idea; how cost- or safety-efficient it might be are separate questions…

Depends on what the alternative is.  Hopefully you're going fairly slow when you get to that point.  In any case, if the choice is between grass and a drop off into a ravine, go for the grass.  If the choice is between grass and steel lighting structures off the end, grass wins again.  If the choice is between grass and special overrun material like sand or EMAS, well in that case grass loses.
How you do on the grass will depend on the surface condition, how dry it is, etc and how fast you are going. If you're going fairly slowly, you won't likely collapse the gear, just make big furrows.  I would prefer to get both mains onto the grass to minimize the risk of going sideways and collapsing the gear, which is what you really want to avoid.
But in any case, you will do better on the turf than going over a cliff or into structures that are going to shred your airplane even at low speed.  If you can see what is beyond the end of the runway as you get there, the choice will be fairly easy.

If the choice is between grass and tarmac, grass also loses, IIRC. That's why the stopping distance on a grass runway is longer than the distance on a paved runway.

@nick012000 Is that based on the brakes working? With brakes, one would would expect to get greater traction, and therefore retardation, on tarmac.

@sdenham "Is that based on the brakes working?" Yes, though I doubt that grass runways would become more effective than paved ones if your brakes aren't working.

@nick012000 That is an independent question, to which the performance with brakes has little obvious relevance.  I would guess that a flat, grassy surface firm enough to not be deformed by the aircraft's weight would be about the same as tarmac, though when we are talking about airliners (as we are), that firmness criterion might not hold very often.

I'm surprised to find that english language seems to lack a proper specifict word for a soil that is composed of decomposed organic matter such as plants. Anyway, a grassy field would have such a base, and it's load bearing capacity is typically quite poor. So tires would sink into it and create noticeable drag.

@Jpe61: Perhaps "turf"?

That's close, but turf generally is the topsoil with grass.

@Jpe61 "humus". Not to be confused with "hummus"..."humus" is not good in a sandwich.

Typically, you have a main braking system, an alternate one and a backup one. Then you have reversers, and runway over run.
The only time i would choose grass (in an airliner) is the one at the end of the runway.
Also, the runway is a better platform for emergency vehicules and passenger evacuation.